How Many Sand Dollars Are There On Sanibel?

At Sanibel Sea School, we've always wondered how many sand dollars are buried under our feet on the sand bars. Sometimes you can't seem to wade through the water without stepping through piles of them and other times, you can search for hours and not find a single specimen! One of our longtime students set out to find the answer to this puzzling question. Check out her cool findings!

By: Alice Toussaint Pittman

My Questions

From the middle of January to the end of February, I studied sand dollar population density off the coast of Sanibel Island, FL. My two big research questions were: on average, how many sand dollars are there per square meter; and, what is their average age, which is believed to correlate with size.

Methods

My method for counting sand dollars included using a quadrat, which is a 1 meter by 1 meter square of PVC pipe; a basket wrapped in a pool noodle; a ruler; and a small waterproof notebook. I would walk out to the sand bar in a specified area on Colony Beach between 2 designated boardwalks. I would then close my eyes and throw the quadrat. Once it landed in the water, I would walk over to it, hold it on the ground with one foot, and twist my other foot all around inside of the quadrat. If I found a sand dollar I would put in the basket. Once I had churned up all the sand in the square I would pick the quadrat up and throw it again. This procedure would be repeated 20 times. After I had collected all the sand dollars, I would mark how many I found, measure them, record all of the numbers in my book, then, throw them back in the water.

Results

I found a total of 93 sand dollars, and most of them were between 84 and 98 mm in diameter. The smallest sand dollar was 71 mm. The largest I found was 110 mm. The average size was around 86 mm. I believe that the sand dollars in the area are almost at their maximum size.

Here is the frequency histogram that shows the size distribution of the sand dollars sampled. You can see that the average size was around 86 mm.

I didn’t answer my population density question because I realized I was using a faulty method. For example, if you wanted to measure the population density of red ants in your backyard, you could throw a quadrat randomly 100 times and end up not landing on either of the 2 highly populated ants nests that exist on the property. If this happened, you may find very few or no ants at all. Your data would seem to reflect that there is a very low or no population density. But that would be incorrect. There are ants, just not evenly distributed. I suspect the same thing happened with me and my sand dollars.

This frequency histogram shows the size distribution of sand dollars using size intervals. This shows that I did not find any sand dollars that were under 69 mm or over 110 mm - possibly indicating that most of the sand dollars were around the same age.

What I Learned

I learned a lot about research trial and error. Sometimes things don’t always work the first time in science and you have to try different methods and procedures to accurately collect the data you are looking for.

I also learned about some of the obstacles that scientists have to work past, like bad weather and high tides. One time this year, we had a big storm and we suspect all the sand dollars got buried under a ton of sand. It was really difficult to sample so we had to postpone the project for a week.

Sampling sand dollars during high tide is easier said than done!

In addition, I learned how data could easily be influenced by a scientist’s bias. For example, I was aware there were a lot of sand dollars in one area and I could have collected more data by always throwing there, but I tried to throw randomly throughout the entire area to give a true sampling. If for some reason my motivation was to show a high or low population density I may subconsciously tend to throw in one direction or another.

Although I did not find the population density, I am happy I was able to set a baseline for further study of the sand dollar life cycles in this area. I believe that the sand dollars in the area are almost at their maximum size. Scientists will be able to use my data to compare with size data from other months and hopefully learn more about the life cycle and size/age correlation of the sand dollar around Sanibel Island.